Curable composition for producing a dental composite crown and process of production

ABSTRACT

The invention relates to A curable composition for producing dental composite crowns, the composition comprising a resin matrix comprising polymerizable (meth)acrylate(s) not comprising a urethane moiety, polymerizable urethane(meth)acrylate(s), wherein the polymerizable (meth)acrylate(s) not comprising an urethane moiety are used in excess over the polymerizable urethane(meth)acrylate(s), a filler matrix comprising nanocluster(s), fumed silica in an amount below 8 wt. % with respect to the weight of the whole composition, an initiator system comprising photoinitiator(s), organic dye(s), the curable composition not comprising softener in an amount of more than 5 wt. % with respect to the weight of the whole composition, the curable composition having a viscosity below 150 Pa*s at 23° C. and a shear rate of 1 s −1 . 
     The invention also relates to a cured article obtained by radiation curing this curable composition by use of an additive-manufacturing method.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/622,530, filed Dec. 13, 2019, now allowed, which is a 35 U.S.C. 371of PCT/US2018/036025, filed Jun. 5, 2018, which claims priority to EP17176014.3, filed Jun. 14, 2017, the disclosure of which is incorporatedby reference in its/their entirety herein.

FIELD OF THE INVENTION

The invention relates to a curable composition suitable for producingdental composite crowns, a prefabricated dental composite crown and aprocess for producing such a dental composite crown by anadditive-manufacturing technique.

BACKGROUND

For treating dental defects a variety of different solutions aremeanwhile on the market. Generally, dental defects can be treated byrestorative methods or prosthetic methods. Direct restoration compositesare for example highly filled materials which are characterized byexcellent mechanical properties and low wear. Unfortunately, due to thehigh filler loading these materials tend to be brittle.

In contrast, temporary crown and bridge materials used as a prostheticmaterial have a lower filler content. This results in an improvedelasticity and a higher fracture resistance, but also in an increasedwear which prevents the long-term use of these materials. For anin-office or chairside fabrication of composite indirect restorationsthere is a need for a material that combines the properties of thematerials described above. Prosthetic methods are typically used, if notsufficient remaining tooth structure is left which allows a restorativetreatment, e.g. by filling a cavity with a dental filling material. Theprosthetic treatment usually starts with taking a dental impression fromthe dental situation in the mouth of the patient. The obtained recordrepresents the status as is.

In a next step, the tooth to be treated is further prepared, i.e. thetooth is shaped to a form which later allows the fixation of anartificial crown. The artificial crown is typically designed from theinformation obtained from the dental impression and the shape of thetreated tooth. In a next step, the artificial crown is produced in adental lab and specifically designed for this individual case. Thisprocedure takes time and is expensive.

If, however, a fast and cheap prosthetic treatment is desired, thepractitioner might consider using preformed crowns instead. This kind oftreatment is often used in pediatric dentistry. Different prefabricatedcrown types are meanwhile available on the market for this purpose.

A popular solution is the use of stainless steel crowns (e.g. from 3MOral Care; 3M ESPE). Stainless steel crowns are easy to manufacture andthey are also durable over years.

Preformed crowns made from stainless steel have in addition the benefitthat they are pre-trimmed, belled and crimped for fast and easyplacement. Due to a so-called “snap-on” feature, the stainless steelcrown is readily retained and fits over the contour of the preparedtooth.

However, due to its metal surface, stainless steel crowns do not meetthe desired aesthetic requirements. To cure this defect, veneeredstainless steel crowns have been suggested.

However, veneered stainless steel crowns show only a limited flexibilitydue to stiffer walls, at least at some areas of the crown. Moreover,chipping of the veneering of these crowns is reported and a metallicshine of the underneath stainless steel crown lowers the estheticappearance. Thus, it was suggested to try to manufacture preformedcrowns out of other materials. Zirconia ceramic (ZrO₂) is quite commonfor individually designed crowns for esthetic dentistry.

ZrO₂ has a couple of unique material properties, e.g. high strength andtoughness, translucency, stainability and biological compatibility,which makes it well suitable for crowns or even bridges.

However, due to material properties, the side walls of these crowns arenot flexible and therefore no undercut design is possible. Further,compared to stainless steel crowns, a different and more invasive toothpreparation without or with a very limited possibility of undercutretention is needed, which leads again to a more careful cementationtechnique. It has also been suggested to manufacture preformed crownsout of polymeric materials.

In this respect, U.S. Pat. No. 8,651,867 B2 (Zilberman) describes adental crown configured to be readily mountable in a patient's mouth aspart of a treatment of primary teeth and permanent molars, the dentalcrown having a natural appearance and colour of a vital tooth andconsisting of a thermoplastic material layer configured to define atooth shaped top surface and flexible side surfaces.

As suitable thermoplastic materials polymers selected from polyacetal,polyacrylate, polymethacrylate (PMMA), polyaryletherketone (PAEK),polyetherketon (PEK), polyetheretherketon (PEEK), polyetherimide (PEI),polyethersulfone (PES) and polysulfone (PSU) are suggested.

US 2004/0161726 A1 (Saito et al.) describes a crown prosthesis havingwear resistance and an aesthetic property comprising a polymer of amixture of a polymerizable compound having an unsaturated double bond, afiller and a polymerization initiator, and having an outer shaperesembling a tooth and a space to be filled with a dental compositeresin between an inner surface thereof and an abutment tooth.

However, the workflow needed for applying polymer material basedpreformed crowns does not really differ from the workflow needed forapplying preformed zirconia crowns, even if a more flexible material isused.

The commercially available polymer based preformed crowns typically showwall thicknesses of more than 500 μm, which gives them not enoughflexibility and therefore no undercut design options as an additionalsupport for cementation.

The fact that the material properties do not allow thinner walls makesthe walls a limiting factor for the design options.

To nevertheless ensure a sufficient adhesive fixation of these crowns toa prepared tooth surface, an adhesive cementation is recommended orrequired. This makes the whole prosthetic procedure more complicated andexpensive. Generally, for crowns and bridges different cementationtechniques are available.

These can be divided into clusters like temporary cementation (e.g.RelyX™ TempNE/E from 3M Oral Care; 3M ESPE), conventional cementation(e.g. Ketac™ CEM or Ketac™ CEM Plus from 3M Oral Care; 3M ESPE),self-adhesive resin cementation (e.g. RelyX™ Unicem from 3M Oral Care;3M ESPE) or adhesive resin cementation (e.g. RelyX™ Ultimate from 3MOral Care; 3M ESPE).

In general, the cementation needs to be durable over the life time ofthe indication, which could be achieved either due to chemical bondingor mechanical retention or a combination thereof.

The choice of the used cement or the general cementation technique for aspecific indication is therefore influenced by the material of therestoration, the indication itself, the preparation technique, but alsocost and esthetic plays a role.

For a fast and easy chairside workflow with preformed crowns, e.g.pediatric dentistry a fast and easy cementation technique is not onlydesired but required.

For this reason, for the fixation of stainless steel crowns conventionalcementation techniques are used. Those cements do not only have aneasier workflow, but also are cheaper than self-adhesive resin oradhesive resin cements. Moreover, they are more moisture tolerant androbust against blood and saliva than self-adhesive or adhesive cements.

This technique is the dominating one in pediatric dentistry, due to thetime saving chairside workflow and the fact, that an individuallydesigned crown is not necessarily needed.

WO 2007/098485 A2 (Nusmile) describes a preformed dental crown with acenter surface, a circumferential surface transitioning from andintegral with the central surface wherein the circumferential surfaceincludes a taper toward a gingival end and wherein said taper has athickness ranging from 0 to 0.5 mm at a gingival edge to at least 1.0 mmproximate the transition to the center surface.

WO 2008/033758 A2 (3M) describes a solid dental crown including aself-supporting solid hardenable preformed dental crown having anexternal crown shape defined by an external crown shape defined by anexternal crown surface.

US 2007/0196792 A1 (Johnson et al.) describes a prefabricated dentalcrown being tooth coloured and having an undercut. Materials which aresaid to be useful for manufacturing the prefabricated dental crown arethermoplastic resins such as polyacetal, polyacrylate, polyamide,polyaryletherketone, polyetheretherketone (PEEK), polyetherimide, etc.

WO 2013/153183 (IvoclarVivadent) describes the use of a composite resincomposition containing: (a) at least one poly-reactive binder, (b) afirst photo-polymerisation initiator with an absorption maximum at awavelength of less than 400 nm, (c) a second photo-polymerisationinitiator with an absorption maximum at a wavelength of at least 400 nm,and (d) an absorber with an absorption maximum at a wavelength of lessthan 400 nm for the stereo-lithographic production of a dental formedcomponent on the basis of composite resin.

US 2014/131908 (Dentsply) describes a composition for making athree-dimensional dental prosthesis comprising a mixture of 1 to 99.5%of monomer, 5 to 99% of at least one mono or multifunctional(meth)acrylate, 0 to 60% of at least one inorganic filler, 0 to 60% ofat least one organic fillers, 5 to 10% a silicone-acrylic-based rubberimpact modifier, 0 to 10% pigments, and 0.01 to 10% of light initiators.

U.S. Pat. No. 8,329,776 (Hecht et al.) describes hardenable dentalcompositions comprises urethane(meth)acrylates, (meth)acrylates,different kind of fillers and initiator.

WO 2015/006087 A1 (3M) describes a hardenable dental compositioncomprising aggregated nano-sized particles, agglomerated nano-sizedparticles, (meth)acrylates, urethane(meth)acrylates and a redox curinginitiator system.

The hardenable dental compositions described in U.S. Pat. No. 8,329,776and WO 2015/006087 are typically provided as kit of parts comprising abase paste and a catalyst paste. The hardenable compositions can be usedin particular for producing artificial crowns and bridges for temporaryor long-term use.

US 2013/0210959 A1 (Yang et al.) describes curable dental compositionscomprising a resin system comprising a free-radically polymerizablesemi-crystalline resin having a molecular weight no greater than 2000g/mol and at least 50 wt. % of nano-cluster filler. According to oneembodiment, the composition is provided as a preformed dental crown inthe form of a hardenable self-supporting structure having a first shapeand sufficient malleability to be formed into a second shape. Theshaping can be done e.g. by extruding, injection molding, pressing andcalendaring.

US 2016/0136059 A1 (Hecht et al.) relates to dental compositionscomprising a filler with aggregated non-sized particles and a fillercomprising agglomerated nano-sized particles, urethane(meth)acrylates,(meth)acrylates and a redox curing initiator system.

US 2014/0131908 A1 (Sun et al.) relates to a printable polymerizablematerial system containing a silicone-acrylic-based rubber impactmodifier, for making e.g. artificial teeth. The polymerizable materialsystem may contain filler in an amount of 0 to 75 wt. %. In the examplesection compositions are described containing filler in an amount ofmore than 55 wt. %.

US 2015/0111176 A1 (Wachter et al.) relates to the use of a compositeresin composition comprising a polyreactive binder, a firstphotopolymerization initiator, a second photopolymerization initiatorand an absorber for the stereolithographic production of a dental shapedpart based on composite resin. The resin composition also comprises afiller. A filler amount in the range of 50 to 80 wt. % is said to bepreferred. As examples compositions are given containing a mixture ofBis-GMA and UDMA in combination with a filler content of more than 60wt. %. However, none of the solutions suggested in the prior art iscompletely satisfying.

All these types of crowns do have drawbacks from different aspects,mainly related to a more complex and time-consuming workflow but alsodue to durability and/or esthetic.

DESCRIPTION OF THE INVENTION

There is a desire for prefabricated composite crowns showing basicallythe same performance as temporary crowns obtained from commerciallyavailable paste/paste materials (e.g. Protemp™ 4; 3M Oral Care), which,however, are in addition autoclavable. Before a particular preformeddental composite crown is finally fixed to the surface of a preparedtooth, it is often required to test various sizes or shapes of differentpreformed dental composite crowns in the mouth of a patient.

After the testing process, the remaining tested preformed dentalcomposite crowns are contaminated and thus need to be autoclaved beforea re-use is possible.

Autoclave conditions typically include the heating of the article in ahumid atmosphere for at least 15 min at a temperature of at least 120°C. During such a process, composite articles containing a redoxinitiator system typically tend to become brittle.

Ideally, the desired prefabricated composite crown should showessentially the same performance before and after an autoclave processhas been performed and the respective mechanical values (like E-modulus)should not deviate by e.g. more than about +/−10% with respect to theoriginal value.

Ideally, the prefabricated composite crowns should show a so-called“snap-on effect” comparable to stainless steel crowns.

Further, the prefabricated composite crowns should be easy tomanufacture, i.e. it should be possible to efficiently and produce asufficient volume of crowns.

One or more of the above objects is addressed by the invention describedin the present text and the claims.

In one embodiment, the invention features a curable composition forproducing dental composite crowns as described in the present text andthe claims, the curable composition comprising:

-   -   a resin matrix comprising:        -   polymerizable (meth)acrylate(s) not comprising a urethane            moiety,        -   polymerizable urethane(meth)acrylate(s),        -   wherein the polymerizable (meth)acrylate(s) not comprising            an urethane moiety are used in excess over the polymerizable            urethane(meth)acrylate(s),    -   a filler matrix comprising:        -   nanocluster(s),        -   optionally fumed silica, preferably in an amount below 8 wt.            %,        -   the filler matrix being present preferably in an amount of 5            to 45 wt. %,    -   an initiator system comprising:        -   photoinitiator(s),        -   organic dye(s),    -   the curable composition not comprising softener in an amount of        more than 5 wt. %, wt. % with respect to the weight of the whole        composition,    -   the curable composition having a viscosity below 150 Pa*s at        23° C. and a shear rate of 1 s⁻¹.

The invention also relates to a dental composite crown obtained bycuring the curable composition as described in the present text and theclaims.

The invention also relates to a process of producing dental compositecrowns with an additive manufacturing technique as described in thepresent text and the claims.

The invention also relates to a kit of parts comprising at least twodifferent dental composite crowns as described in the present text andthe claims.

Unless defined differently, for this description the following termsshall have the given meaning:

A “hardenable component or material” or “polymerizable component” is anycomponent which can be cured or solidified by radiation-inducedpolymerization or crosslinking. A hardenable component may contain onlyone, two, three or more polymerizable groups. Typical examples ofpolymerizable groups include unsaturated carbon groups, such as a vinylgroup being present i.a. in a (methyl)acrylate group.

A “photoinitiator” is a substance being able to start or initiate thecuring process of a polymerizable composition upon exposure to radiation(e.g. wavelength of 350 to 600 nm or 350 to 420 nm).

A “monomer” is any chemical substance which can be characterized by achemical formula, bearing polymerizable groups (including (meth)acrylategroups) which can be polymerized to oligomers or polymers therebyincreasing the molecular weight. The molecular weight of monomers canusually simply be calculated based on the chemical formula given.

As used herein, “(meth)acryl” is a shorthand term referring to “acryl”and/or “methacryl”. For example, a “(meth) acryloxy” group is ashorthand term referring to either an acryloxy group (i. e.,CH₂═CH—C(O)—O—) and/or a methacryloxy group (i. e., CH₂═C(CH₃)—C(O)—O—).

A “curing, hardening or setting reaction” is used interchangeable andrefers to a reaction, wherein physical properties such as viscosity andhardness of a composition changes over the time due to a chemicalreaction between the individual components. A “urethane group” is agroup having the structure “—NH—CO—O—”.

The term “dental or orthodontic article” means any article which is tobe used in the dental or orthodontic field, especially for producing adental restoration, orthodontic devices, a tooth model and partsthereof.

Examples of dental articles include crowns, bridges, inlays, onlays,veneers, facings, copings, crown and bridged framework, implants,abutments, dental milling blocks, monolithic dental restorations andparts thereof.

Examples of orthodontic articles include brackets, buccal tubes, cleatsand buttons and parts thereof.

A dental or orthodontic article should not contain components which aredetrimental to the patient's health and thus free of hazardous and toxiccomponents being able to migrate out of the dental or orthodonticarticle. The surface of a tooth is considered not to be a dental ororthodontic article.

“Glass” means an inorganic non-metallic amorphous material which isthermodynamically an under-cooled and frozen melt. Glass refers to ahard, brittle, transparent solid. Typical examples include soda-limeglass and borosilicate glass. A glass is an inorganic product of fusionwhich has been cooled to a rigid condition without crystallizing. Mostglasses contain silica as their main component and a certain amount ofglass former. The material or article described in the present text doestypically not contain a glass.

“Glass-ceramic” means an inorganic non-metallic material where one ormore crystalline phases are surrounded by a glassy phase so that thematerial comprises a glass material and a ceramic material in acombination or mixture. Thus, a glass ceramic is a material sharing manyproperties with both glass and more traditional crystalline ceramics. Itis formed as a glass, and then made to crystallize partly by heattreatment. Glass ceramics may refer to a mixture of lithium-, silicon-and aluminium oxides.

A “powder” means a dry, bulk material composed of a large number of fineparticles that may flow freely when shaken or tilted.

A “particle” means a substance being a solid having a shape which can begeometrically determined. The shape can be regular or irregular.Particles can typically be analysed with respect to e.g. particle sizeand particle size distribution. A particle can comprise one or morecrystallites. Thus, a particle can comprise one or more crystal phases.

The term “associated” refers to a grouping of two or more primaryparticles that are aggregated and/or agglomerated.

Similarly, the term “non-associated” refers to two or more primaryparticles that are free or substantially free from aggregation and/oragglomeration.

“Aggregated,” as used herein, is descriptive of a strong association ofparticles often bound together by, for example, residual chemicalstreatment or partially sintering. The specific surface of aggregatedparticles is typically smaller than the specific surface of the primaryparticles the aggregate is made of (cf. DIN 53206; 1972).

Further breakdown of the aggregates into smaller entities may occurduring a polishing step applied to the surface of a compositioncontaining the aggregated filler but not during dispersing theaggregated particles in a resin.

Aggregated fillers and processes for the production and surfacetreatment thereof are described e.g. in WO 01/30304 and U.S. Pat. No.6,730,156 (3M). The content of these references is herewith incorporatedby reference.

“Agglomerated” is descriptive of a weak association of particles usuallyheld together by charge or polarity and can be broken down into smallerentities.

Agglomerated fillers are commercially available e.g. from Degussa, CabotCorp or Wacker under the product designation Aerosil™, CAB-O-SIL™ andHDK.

A “non-agglomerated filler” means that the filler particles are presentin the resin in a discrete, un-associated (i.e. non-agglomerated andnon-aggregated) stage. If desired this can be proven by TEM microscopy.

Non-agglomerated nano-sized silicas are commercially available e.g. fromNalco Chemical Co. (Naperville, Ill.) under the product designationNALCO COLLOIDAL SILICAS e.g. NALCO products #1040, 1042, 1050, 1060,2327 and 2329.

Non-agglomerated fillers are used and described e.g. in EP 2 167 013 B1(3M). The content of this reference is herewith incorporated byreference.

The term “primary particle size” refers to the size of a non-associatedsingle crystal zirconia particle, which is considered to be a primaryparticle. X-ray diffraction (XRD) is typically used to measure theprimary particle size.

The mean particle size of a powder can be obtained from the cumulativecurve of the grain size distribution and is defined as the arithmeticaverage of the measured grain sizes of a certain powder mixture.Respective measurements can be done using commercially availablegranulometers (e.g. CILAS Laser Diffraction Particle Size AnalysisInstrument).

A “nano-filler” is a filler, the individual particles thereof have asize in the region of nanometers, e.g. an average particle diameter ofless than about 200 nm or less than about 100 nm or less than about 50nm. Useful examples are given in U.S. Pat. No. 6,899,948/Zhang et al.)and U.S. Pat. No. 6,572,693 (Wu et al.). The content with regard tonano-sized silica particles is herein incorporated by reference.

The measurement of the size of nano-particles is preferably based on aTEM (transmission electron microscopy) method, whereby a population isanalyzed to obtain an average particle diameter. A preferred method formeasuring the particle diameter can be described as follows:

Samples with a thickness not exceeding 80 nm are placed on 200 meshcopper grids with carbon stabilized formvar substrates (SPI Supplies—adivision of Structure Probe, Inc., West Chester, PA). A transmissionelectron micrograph (TEM) is taken, using JEOL 200CX (JEOL, Ltd. ofAkishima, Japan and sold by JEOL USA, Inc.) at 200 KV. A population sizeof about 50-100 particles can be measured and an average diameter isdetermined.

“Additive manufacturing” means processes used to make 3-dimensionalarticles. An example of an additive manufacturing technique isstereolithography (SLA) in which successive layers of material are laiddown under computer control and are subsequently cured by radiation. Thearticles can be of almost any shape or geometry and are produced from a3-dimensional model or other electronic data source. Other examples ofadditive manufacturing processes or techniques include 3d-printing.

“Resin modified glass ionomer cement” means a hardenable dental materialcomprising acid-reactive glass, polyacid, water, polymerizablecomponents and initiator. Resin modified glass ionomer cements undergo atwofold curing reaction, a glass ionomer acid base based cement reactionand polymerization of typically (methacrylate) acrylate based monomers.

“Adhesive resin cement” means a hardenable dental material which curesby radical polymerization of polymerizable components (but not by aglass ionomer cement reaction). An adhesive resin cement requires apre-treatment of the hard dental surfaces to effect adhesion. Incontrast to resin modified glass ionomer cements, an adhesive resincement does not contain added water.

A “self-adhesive resin cement” is an adhesive resin cement which inaddition contains acidic components and thus does not require apre-treatment of the hard dental surfaces to effect adhesion.

In contrast to resin modified glass ionomer cements, adhesive resincement and self-adhesive resin cement typically only cure bypolymerization reaction.

“Density” means the ratio of mass to volume of an object. The unit ofdensity is typically g/cm³. The density of an object can be calculatede.g. by determining its volume (e.g. by calculation or applying theArchimedes principle or method) and measuring its mass.

The volume of a sample can be determined based on the overall outerdimensions of the sample. The density of the sample can be calculatedfrom the measured sample volume and the sample mass. The total volume ofa material sample can be calculated from the mass of the sample and thedensity of the used material. The total volume of cells in the sample isassumed to be the remainder of the sample volume (100% minus the totalvolume of material).

A material or composition is “essentially or substantially free of” acertain component within the meaning of the invention, if the materialor composition does not contain said component as an essential feature.Thus, said component is not wilfully added to the composition ormaterial either as such or in combination with other components oringredient of other components. A composition or material beingessentially free of a certain component usually contains the componentin an amount of less than 1 wt. % or less than 0.1 wt. % or less than0.01 wt. % (or less than 0.05 mol/1 solvent or less than 0.005 mol/1solvent or less than 0.0005 mol/1 solvent) with respect to the wholecomposition or material. Ideally the composition or material does notcontain the said component at all. However, sometimes the presence of asmall amount of the said component is not avoidable e.g. due toimpurities.

“Ambient conditions” mean the conditions which the inventive compositionis usually subjected to during storage and handling. Ambient conditionsmay, for example, be a pressure of 900 to 1100 mbar, a temperature of−10 to 60° C. and a relative humidity of 10 to 100%. In the laboratoryambient conditions are adjusted to about 23° C. and about 1013 mbar. Inthe dental and orthodontic field ambient conditions are reasonablyunderstood as a pressure of 950 to 1050 mbar, temperature of 15 to 40°C. and relative humidity of 20 to 80%.

As used herein, “a”, “an”, “the”, “at least one” and “one or more” areused interchangeably. The terms “comprise” or “contain” and variationsthereof do not have a limiting meaning where these terms appear in thedescription and claims. Also herein, the recitations of numerical rangesby endpoints include all numbers subsumed within that range (e.g., 1 to5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

Adding an “(s)” to a term means that the term should include thesingular and plural form. E.g. the term “additive(s)” means one additiveand more additives (e.g. 2, 3, 4, etc.).

Unless otherwise indicated, all numbers expressing quantities ofingredients, measurement of physical properties such as described belowand so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about”. The term“comprise” shall include also the terms “consist essentially of” and“consists of”.

DETAILED DESCRIPTION

The invention described in the present text is advantageous for a coupleof reasons. The curable composition described in the present text can beeasily processed as construction material in an additive manufacturingprocess comprising a radiation step, in particular using astereolithographie technique (SLA technique).

Without wishing to be bound to a certain theory, it is believed that theviscosity of the curable composition contributes to an easy processingof the composition in an additive-manufacturing technique.

This allows also the simultaneous production of more individualizedcomposite crowns.

Further, without wishing to be bound to a certain theory, it is believedthat due to the limited amount or even absence of softeners, the curedcomposition becomes better autoclavable.

Softeners are typically present in curable composition for use astemporary crown and bridge materials which are provided as kit of partscomprising a base paste and a catalyst paste. These softeners maymigrate out of the cured composite article during an autoclave process.

As the curable composition described in the present text does typicallynot contain softener or only in small amounts, the risk that softenersmigrate out from the cured composition during an autoclave process isreduced.

Nevertheless, it was found that the mechanical properties of the curedcomposition remain fairly unchanged after an autoclaving has beenconducted.

Further, the obtained printed article often shows less defects andinhomogeneity compared to articles obtained by curing a redox-initiatorcontaining curable composition, which has been provided by mixing acatalyst and base paste either by hand or by using a static mixing tip.

The curable composition described in the present text enables theproduction of 3-dim articles or products with precise build features andsmooth surfaces.

It was also found that the curable composition described in the presenttext allows the production of 3-dimensional structures with wallthicknesses below 0.8 mm.

Further, the curable composition described in the present text allowsthe production of prefabricated composite crowns having the desired“snap-on effect” comparable to stainless steel crowns.

Thus, the article obtained after having processed the curablecomposition as construction material in an additive manufacturingprocess is characterized by a combination of specific properties such ashigh mechanical strength, high fracture resistance and high aesthetics.

The curable composition is in particular useful in the dental andorthodontic field and may also facilitates the chair-side production ofindirect dental restorations on the same day.

Examples of such indirect dental restorations include dental crowns andbridges, in particular preformed dental composite crowns having a shapewhich has an undercut structure.

Such a shape provides the crown with a so-called “snap-on effect” andallows an easy placement of the preformed composite crown on the surfaceof a prepared tooth, in particular in the pediatric field.

In certain embodiments, the curable composition fulfils one or more,sometimes all of the following properties:

-   -   a) curable by radiation having a wavelength of in the range of        350 to 600 nm or 350 to 420 nm;    -   b) viscosity: 1 to 100 Pa*s at 23° C. at a shear rate of 1 s⁻¹;    -   c) pH value: 6 to 8, if brought in contact with wet pH sensitive        paper.

If desired, the properties can be measured as described in the examplesection.

In certain embodiments, the combination of the following features issometimes desirable: a) and b), or a), b) and c).

The curable composition described in the present text is radiationcurable in a wave length which is typically used in commerciallyavailable additive manufacturing equipment.

Further, the curable composition described in the present text typicallyhas a viscosity which allows the processing of the composition in an SLAprocess. A lower viscosity often allows a better printing quality inparticular as regards surface accuracy.

The curable composition described in the present text is typicallyopaque.

As the curable composition does typically not contain acidic components,the pH value of the composition is in the neutral range.

The curable composition described in the present text comprises a resinmatrix.

The resin matrix typically comprises 40 to 90 wt. % or 50 to 80 wt. % ofthe curable composition.

The resin matrix comprises polymerizable (meth)acrylate(s) notcomprising a urethane moiety as component (A) and polymerizableurethane(meth)acrylate(s) as component (B).

The resin of the curable composition also comprises a (meth)acrylate(s)having at least 1 or 2 polymerizable moieties but not comprising aurethane moiety.

Thus, the (meth)acrylate(s) is different from urethane(meth)acrylate,e.g. with respect to functionality, chemical moieties, molecular weightor combinations thereof.

If desired, the chemical composition may comprise at least two, three orfour different kinds of (meth)acrylate(s).

Adding a (meth)acrylate to the resin composition helps to furtherimprove the mechanical properties of the resin composition in its curedstage, in particular with regards to flexural strength or abrasionresistance.

The molecular weight of the (meth)acrylate(s) is typically at least 170or at least 200 or at least 300 g/mol.

The molecular weight of the (meth)acrylate(s) is typically in a range of170 to 3,000 or 200 to 2,500 or 300 to 2,000 g/mol.

The (meth)acrylate(s) has free radically active functional groups andincludes monomers, oligomers, and polymers having two or moreethylenically unsaturated groups.

Such free radically polymerizable materials include di- orpoly-acrylates and methacrylates such glycerol diacrylate, glyceroltriacrylate, ethyleneglycol diacrylate, diethyleneglycol diacrylate,triethyleneglycol dimethacrylate, 1,3-propanediol diacrylate,1,3-propanediol dimethacrylate, trimethylolpropane triacrylate,1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate,pentaerythritol triacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, sorbitol hexacrylate,bis[1-(2-acryloxy)]-p-ethoxyphenyldimethylmethane,bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethyl-methane; thebis-acrylates and bis-methacrylates of polyethylene glycols of molecularweight 200-500, copolymerizable mixtures of acrylated monomers such asthose in U.S. Pat. No. 4,652,274, and acrylated oligomers such as thoseof U.S. Pat. No. 4,642,126; and vinyl compounds such as diallylphthalate, divinyl succinate, divinyl adipate and divinylphthalate.

Preferred ethylenically unsaturated monomers are methacrylate andacrylate monomers, such as di(meth)acrylates of propanediol, butanediol,hexanediol, octanediol, nonanediol, decanediol and eicosanediol,di(meth)acrylates of ethylene glycol, of polyethylene glycols and ofpolypropylene glycols, di(meth)acrylates of ethoxylated bisphenol A, forexample 2,2′-bis(4-(meth)acryloxytetraethoxyphenyl)propanes, and(meth)acrylamides. The monomers used can furthermore be esters of[alpha]-cyanoacrylic acid, crotonic acid, cinnamic acid and sorbic acid.

It is also possible to use the methacrylic esters mentioned in EP 0 235826 A1 (ESPE), such as bis[3[4]-methacryl-oxymethyl-8(9)-tricyclo[5.2.1.0^(2,6)]decylmethyltriglycolate. Particularly suitable are 2,2-bis-4(3-methacryloxy-2-hydroxypropoxy)phenylpropane (Bis-GMA), 2,2-bis-4(3-methacryloxypropoxy)-phenylpropane, triethylene glycoldimethacrylate (TEGDMA), and di(meth)acrylates ofbishydroxymethyltricyclo-(5.2.1.0^(2,6))decane.

It was found that using (meth)acrylate(s) and more particularly, thecomponents described above, can be beneficial to provide the hardenedcomposition with sufficient mechanical strength as it may function as akind of crosslinking agent useful for improving the mechanicalproperties of the cured dental composition.

The (meth)acrylate(s) is typically present in the following amounts:

-   -   lower amount: at least 40 or at least 45 or at least 50 wt. %;    -   upper amount: utmost 85 or utmost 80 or utmost 70 wt. %;    -   range: 40 to 85 or 45 to 80 or 50 to 70 wt. %;        wt. % with respect to the weight of the polymerizable        composition.

The polymerizable (meth)acrylates not containing a urethane moiety areused in excess compared to the polymerizable urethane(meth)acrylates.

The following ratio was found to be suitable:

[polymerizable (meth)acrylates not containing a urethanemoiety]/[polymerizable urethane (meth)acrylates] from 5:1 to 1.5:1.

The curable composition comprises at least one, two, three or fourdifferent kinds of urethane(meth)acrylate(s).

It was found that the addition of urethane(meth)acrylate(s) to the resinmatrix contributes to improving certain mechanical properties likeE-modulus and fracture work of the cured composition. The molecularweight of the urethane(meth)acrylate is typically at least 450 or atleast 800 or at least 1,000 g/mol. Useful ranges include 450 to 3,000 or800 to 2,700 or 1,000 to 2,500 g/mol.

Molecules having a molecular weight above 450 g/mol or above 1,000 g/molare usually less volatile than molecules having a lower molecular weightand thus may contribute to providing a biocompatible composition.

Further, a sufficiently high molecular weight of theurethane(meth)acrylate may contribute to the desired fracture work ofthe composition after hardening.

The nature and structure of the urethane(meth)acrylate is notparticularly limited unless the desired result cannot be achieved.

Urethane (meth)acrylates may be obtained by a number of processes knownto the skilled person.

The urethane(meth)acrylates employed in the composition are typicallyobtained by reacting an NCO-terminated compound with a suitablemonofunctional (meth)acrylate monomer such as hydroxyethyl acrylate,hydroxyethyl methacrylate, hydroxypropylmethacrylate, preferablyhydroxyethyl- and hydroxypropylmethacrylate.

For example, a polyisocyanate and a polyol may be reacted to form anisocyanate-terminated urethane prepolymer that is subsequently reactedwith a (meth)acrylate such as 2-hydroxy ethyl(meth)acrylate. These typesof reactions may be conducted at room temperature or higher temperature,optionally in the presence of catalysts such as tin catalysts, tertiaryamines and the like.

Polyisocyanates which can be employed to form isocyanate-functionalurethane prepolymers can be any organic isocyanate having at least twofree isocyanate groups. Included are aliphatic cycloaliphatic, aromaticand araliphatic isocyanates.

Any of the known polyisocyanates such as alkyl and alkylenepolyisocyanates, cycloalkyl and cycloalkylene polyisocyanates, andcombinations such as alkylene and cycloalkylene polyisocyanates can beemployed.

Preferably, diisocyanates having the formula X(NCO)₂ are used, with Xrepresenting an aliphatic hydrocarbon radical with 2 to 12 C atoms, acycloaliphatic hydrocarbon radical with 5 to 18 C atoms, an aromatichydrocarbon radical with 6 to 16 C atoms and/or an araliphatichydrocarbon radical with 7 to 15 C atoms.

Examples of suitable polyisocyanates include2,2,4-trimethylhexamethylene-1,6-diisocyanate,hexamethylene-1,6-diisocyanate (HDI), cyclohexyl-1,4-diisocyanate,4,4′methylene-bis(cyclohexyl isocyanate),1,1′-methylenebis(4-isocyanato) cyclohexane, isophorone diisocyanate,4,4′-methylene diphenyl diisocyanate, 1,4-tetramethylene diisocycanate,meta- and para-tetramethylxylene diisocycanate, 1,4-phenylenediisocycanate, 2,6- and 2,4-toluene diisocycanate, 1,5-naphthylenediisocycanate, 2,4′ and 4,4′-diphenylmethane diisocycanate and mixturesthereof.

It is also possible to use higher-functional polyisocyanates known frompolyurethane chemistry or else modified polyisocyanates, for examplecontaining carbodiimide groups, allophanate groups, isocyanurate groupsand/or biuret groups. Particularly preferred isocyanates are isophoronediisocyanate, 2,4,4-trimethyl-hexamethylene diisocyanate andhigher-functional polyisocyanates with isocyanurate structure.

The isocyanate terminated urethane compound is capped with a(meth)acrylate to produce a urethane(meth)acrylate compound. In general,any (meth)acrylate-type capping agent having a terminal hydroxyl groupand also having an acrylic or methacrylic moiety can be employed, withthe methacrylic moiety being preferred.

Examples of suitable capping agents include2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, glyceroldi(meth)acrylate and/or trimethylolpropane di(meth)acrylate.Particularly preferred are 2-hydroxyethyl methacrylate (HEMA) and/or2-hydroxyethyl acrylate (HEA).

The equivalence ratio of isocyanate groups to compounds reactivevis-à-vis isocyanate groups is 1.1:1 to 8:1, preferably 1.5:1 to 4:1.

The isocyanate polyaddition reaction can take place in the presence ofcatalysts known from polyurethane chemistry, for example organotincompounds such as dibutyltin dilaurate or amine catalysts such asdiazabicyclo[2.2.2]octane. Furthermore, the synthesis can take placeboth in the melt or in a suitable solvent which can be added before orduring the prepolymer preparation. Suitable solvents are for exampleacetone, 2-butanone, tetrahydrofurane, dioxane, dimethylformamide,N-methyl-2-pyrrolidone (NMP), ethyl acetate, alkyl ethers of ethyleneand propylene glycol and aromatic hydrocarbons. The use of ethyl acetateas solvent is particularly preferred.

Suitable examples of urethane (meth)acrylates include7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-dioxy-dimethacrylate(e.g. Plex 666-1, Rohm),7,7,9-trimethyl-4,13-dioxo-5,12-diazahexadecane-1,16-dioxy-dimethacrylate(UDMA), urethane (methacrylates) derived from 1,4 and1,3-Bis(1-isocyanato-1-methylethyl)bezene (e.g. as described in EP0934926 A1) and mixtures thereof.

According to one embodiment, the urethane(meth)acrylate is characterizedas follows:

-   -   having the structure A-(-S1-U-S2-MA)_(n), with    -   A being a connector element comprising at least one unit,    -   S1 being a spacergroup comprising at least 4 units connected        with each other,    -   S2 being a spacergroup comprising at least 4 units connected        with each other,        -   the units of A, S1 and S2 being independently selected from            CH₃—, —CH₂—, —O—, —S—, —NR¹—, —CO—, —CR¹═,

—N═, —CR¹R²—,with R¹ and R² being independently selected from hydrogen, alkyl,substituted alkyl, alkenyl, cycloalkyl, substituted cycloalkyl,arylalkyl, aryl or substituted aryl, wherein these units can formlinear, branched or cyclic structures such as alkyl, cycloalkyl, aryl,ester, urethane or amide groups,

-   -   U being a urethane group connecting spacergroups S1 and S2,    -   MA being an acrylate or methacrylate group and    -   n being 3 to 6.

According to one embodiment the urethane(meth)acrylate is represented bythe structure

A-(-S1-U-S2-MA)_(n)

with

-   -   A being a connector element comprising at least about 1, 2, 3,        4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20        units,    -   S1 being a spacergroup comprised of units connected with each        other and comprising at least about 4, 5, 6, 7, 8, 9 or 10        units,    -   S2 being a spacergroup comprised of units connected with each        other and comprising at least about 4, 5, 6, 7, 8, 9, 10, 12,        15, 20 or 25 units,    -   U being a urethane group connecting spacergroups S1 and S2,    -   MA being an acrylate or methacrylate group and    -   n being 3 to 6 or 4 to 6 or 5 to 6.

It can be preferred, if A has a cyclic structure and comprises at leastabout 6 units.

It can further be preferred, if S1 has a linear or branched structureand comprises at least about 4 or about 6 units.

It can further be preferred, if S2 has a linear or branched structureand comprises at least about 6 or about 8 units.

A urethane(meth)acrylate wherein A has a cyclic structure and comprisesat least about 6 units and S1 has a linear structure and comprises atleast about 4 units and S2 has a linear structure and comprises at leastabout 8 units and U is a urethane group can also be preferred.

Neither the atoms of the urethane group connecting S1 and S2 nor theatoms of the (meth)acrylgroup belong to the spacergroup S1 or S2. Thus,the atoms of the urethane group do not count as units of thespacergroups S1 or S2.

The nature and structure of the connector element is not particularlylimited. The connector element can contain saturated (no double bonds)or unsaturated (at least one or two double bonds) units, aromatic orhetero aromatic units (aromatic structure containing atoms including N,O and S).

Specific examples of connector element A having a cyclic structureinclude:

Specific examples of connector element A having a non-cyclic butbranched structure include:

The dotted lines indicate the bondings to the spacergroup S1.

the nature and structure of the spacergroups S1 or S2 is notparticularly limited, either.

The spacergroups are comprised of units connected with each other.Typical units include: CH₃—, —CH₂—, —O—, —S—, —NR¹—, —CO—, —CR¹═,

N═, —CR¹R²—, with R¹ and R² independently selected from hydrogen, alkyl,substituted alkyl, alkenyl, cycloalkyl, substituted cycloalkyl,arylalkyl, aryl or substituted aryl.

These units can form linear, branched or cyclic structures such asalkyl, cycloalkyl, aryl, ester, urethane or amide groups.

The structure of S1 can be identical to the structure of S2. However, insome embodiments the structure of S1 is different from S2. In a specificembodiment the number of units being present in S1 is less or equal thanthe number of units being present in S2.

In a specific embodiment, S1 may have a saturated hydrocarbon structure.

In another specific embodiment, S2 may have a saturated hydrocarbonstructure.

Typical examples of useful spacer groups for S1 include:

the dotted lines indicate the chemical bonding to either the group A orthe group U.

Typical examples of useful spacer groups for S2 include:

The dotted lines indicate the chemical bonding to either the(meth)acrylate group or the group U. The number of the units to becounted according to the invention is given in brackets.

Specific examples of hardenable component (B) include:

Further suitable urethane(meth)acrylates are based on α,ω-terminatedpoly(meth)acrylatdiols (e.g. as described in EP 1 242 493 B1) or can bea polyester, polyether, polybutadiene or polycarbonateurethane(meth)acrylate (e.g. as described in U.S. Pat. No. 6,936,642B2).

The urethane(meth)acrylate is typically present in the followingamounts:

-   -   Lower amount: at least 1 or at least 3 or at least 5 wt. %;    -   Upper amount: utmost 35 or utmost 30 or utmost 25 wt. %;    -   Range: 1 to 35 or 3 to 30 or 5 to 25 wt. %;        wt. % with respect to the weight of the polymerizable        composition.

If the amount of urethane(meth)acrylate is too high, the resultingmaterial might become too flexible and will probably not maintain itsdesired shape.

If the amount of urethane(meth)acrylate is too low, the resultingmaterial might become too brittle. Fracture work and impact strengthmight be negatively affected.

The curable composition described in the present text comprises a fillermatrix.

The filler matrix is typically present in an amount from 5 to 45 wt. %or from 10 to 40 wt. %.

The amount of filler used may have an impact on the viscosity of thecurable composition, the abrasion resistance of the cured composition orboth.

The filler matrix may comprise fumed silica.

The specific surface of the hydrophobic fumed silica is typically from100 to 300 or from 150 to 250 m2/g.

A mixture of different fumed silica can be used, if desired. E.g. amixture of fumed silica the surface of which has been treated with ahydrophobic surface treating agent and fumed silica the surface of whichhas been treated with a hydrophilic surface treating agent can be used.

Suitable hydrophobic surface-treating agents include: —OSiR₃, with Rbeing selected from Ci-4 alkyl, preferably methyl and mixtures thereof.

Hydrophobic fumed silica is also commercially available under the tradedesignations HDK, in particular HDK-H™ 2000 (Wacker), or Aerosil™ R812(Evonik).

It was found that using fumed silica the surface of which has beentreated with surface treating agents containing polymerizable moieties,like (meth)acrylsilanes, may sometimes lead to a non-desired thickeningof the curable composition, which may make the curable composition lesssuitable as processing material in an additive manufacturing process.

Thus, according to one embodiment the curable composition described inthe present text does typically not contain fumed silica having beensurface treated with surface treating agents containing polymerizablemoieties like (meth)acrylsilanes, in an amount of more than 2 wt. % ormore than 1.5 wt. % or more than 1 wt. %.

If present, fumed silica is typically present in either of the followingamounts:

-   -   Lower amount: at least 0.5 or at least 1 or at least 1.5 wt. %;    -   Higher amount: utmost 8 or utmost 7 or utmost 5 wt. %;    -   Range: 0.5 to 8 or 1 to 7 or 1.5 to 5 wt. %;    -   wt. % with respect to the weight of the whole curable        composition.

The filler matrix comprises nanocluster(s). One or more different kindsof nanocluster(s) can be present.

It was found that compared to other fillers, using nanocluster(s) can bebeneficial because it allows for the formulation of a composition withhigh filler load resulting in better mechanical properties, e.g.polishability or abrasion and in higher aesthetics.

The nanocluster can typically be characterized by at least one or all ofthe following features:

-   -   Specific surface: 30 to 400 or 60 to 300 or 80 to 250 m2/g,    -   comprising particles of SiO₂, ZrO₂, Al₂O₃ and mixtures thereof.

If desired, the specific surface can be determined according toBrunauer, Emmet and Teller (BET) by using a device (Monosorb™) availablefrom Quantachrome.

If desired, the mean particle size can be determined by light scatteringusing e.g. a Malvern Mastersizer 2000 device available from MalvernInstruments.

A suitable nano-filler comprising aggregated nano-sized particles can beproduced according to the processes described e.g. in U.S. Pat. No.6,730,156 (preparatory example A).

A useful nano-filler comprising aggregated nano-sized particles can beprepared from a suitable sol and one or more oxygen containing heavymetal compound solution(s) precursors which may be salts, sols,solutions, or nano-sized particles; of these, sols are preferred. Forpurposes of this invention, a sol is defined as a stable dispersion ofcolloidal solid particles within a liquid. The solid particles aretypically denser than the surrounding liquid and small enough so thatthe dispersion forces are greater than the gravitational force. Inaddition, the particles are of a size small enough so that theygenerally do not refract visible light. Judicious choice of theprecursor sols leads to desired degree of visual opacity, strength etc.Factors that will guide the choice of the sol depends on the combinationof the following properties: a) the average size of the individualparticles, which is preferably less than 100 nm in diameter, b) theacidity: the pH of the sol should be preferably below 6 and morepreferably below 4, and c) the sol should be free of impurities thatcause undue aggregation (during the filler preparation process) of theindividual discrete particles, during the subsequent steps such as spraydrying or calcining, into larger size particles that cannot be easilydispersed or commuted and hence decrease the translucency andpolishability of a dental restoration made out of a composite comprisingsuch nanoparticles.

If the starting sol is basic, it should be acidified e.g. by addition ofnitric or other suitable acid to decrease the pH. However, choosing abasic starting sol is less desirable since it requires an additionalstep and may lead to the introduction of undesired impurities. Typicalimpurities that are preferably avoided are metal salts, particularlysalts of alkaline metals e.g. sodium.

The non-heavy metal sol and heavy metal oxide precursors are mixedtogether preferably at a molar ratio to match the index of refraction ofthe hardenable resin. This imparts a low and desirable visual opacity.Preferably, the molar ratio ranges of non-heavy metal oxide (“non-HMO”)to heavy metal oxide (“HMO”), expressed as non-HMO:HMO is 0.5:1 to 10:1,more preferably 3:1 to 9:1, and most preferable 4:1 to 7:1.

In a preferred embodiment where the aggregated nano-sized particlescontain silica and zirconium containing compounds, the method ofpreparation starts with a mixture of silica sol and zirconyl acetate, atabout a 5.5:1 molar ratio.

Prior to mixing the non-heavy metal oxide sol with the heavy metal oxideprecursor, the pH of the non-heavy metal oxide sol is preferably reducedto provide an acidic solution having a pH of 1.5 to 4.0.

The non-heavy metal oxide sol is then slowly mixed with the solutioncontaining the heavy metal oxide precursor and vigorously agitated.Strong agitation is preferably performed throughout the blendingprocess. The solution is then dried to remove the water and othervolatile components. Drying can be accomplished in various ways,including for example, tray drying, fluidized bed and spray drying. Inthe preferred method where zirconyl acetate is used, drying by means ofspray drying.

The resulting dried material is preferably made up of smallsubstantially spherical particles as well as broken hollow spheres.These fragments are then batch calcined to further remove residualorganics. The removal of the residual organics allows the filler tobecome more brittle, which results in more efficient particle sizereduction. During calcining, the soak temperature is preferably set at200° C. to 800° C., more preferably 300° C. to 600° C. Soaking isperformed for 0.5 hours to 8 hours, depending on the amount of materialbeing calcined. It is preferred that the soak time of the calcine stepbe such that a plateaued surface area is obtained. It is preferred thatthe time and temperature be chosen such that the resulting filler iswhite in colour, free from black, grey, or amber coloured particles, asdetermined by visual inspection.

The calcined material is then preferably milled to a median particlesize of less than 5 μm, preferably less than 2 μm (on a volumetricbasis), as can be determined by using a Sedigraph 5100 (Micrometrics,Norcross, Ga.). The particle size determination can be performed byfirst obtaining the specific density of the filler using an Accuracy1330 Pycometer (Micrometrics, Norcross, Ga.). Milling can beaccomplished by various methods including for example, stirred milling,vibratory milling, fluid energy milling, jet milling and ball milling.Ball milling is the preferred method.

The resulting fillers comprise, contain, consist essentially or consistof aggregated nano-sized particles. If desired, this can be proven bytransmission electron microscopy (TEM).

If desired, the surface of the filler particles can be surface treated.The surface-treatment can be accomplished according to a process asdescribed in U.S. Pat. No. 6,730,156 (Windisch et al.) or U.S. Pat. No.6,730,156 (Wu et al.). The content of these references is herewithincorporated by reference.

Once dispersed in the resin, the filler remains in an aggregated stage.That is, during the dispersion step the particles do not break up intodiscrete (i.e. individual) and un-associated (i.e. non-aggregated)particles.

If present, the nanocluster(s) is typically present in either of thefollowing amounts:

-   -   Lower amount: at least 5 or at least 10 or at least 15 wt. %;    -   Higher amount: utmost 40 or utmost 38 or utmost 35 wt. %;    -   Range: 5 to 40 or 10 to 38 or 15 to 35 wt. %;    -   wt. % with respect to the weight of the whole curable        composition.

The curable composition may also comprise x-ray visible particles.

Adding x-ray visible particles to the dental composition is beneficialin that it enables the practitioner to better identify the material ifplaced in the mouth of a patient and distinguish between sound dentaltooth structure and the artificial material. The material becomesradiopaque.

Suitable x-ray visible particles include particles of metal oxides andmetal fluorides. Oxides or fluorides of heavy metals having an atomicnumber greater than about 28 can be preferred. The heavy metal oxide orfluoride should be chosen such that undesirable colours or shading arenot imparted to the hardened resin in which it is dispersed. Forexample, iron and cobalt would not be favoured, as they impart dark andcontrasting colours to the neutral tooth colour of the dental material.More preferably, the heavy metal oxide or fluoride is an oxide orfluoride of metals having an atomic number greater than 30. Suitablemetal oxides are the oxides of yttrium, strontium, barium, zirconium,hafnium, niobium, tantalum, tungsten, bismuth, molybdenum, tin, zinc,lanthanide elements (i.e. elements having atomic numbers ranging from 57to 71, inclusive), cerium and combinations thereof. Suitable metalfluorides are e.g. yttriumtrifluoride and ytterbiumtrifluoride. Mostpreferably, the oxides and fluorides of heavy metals having an atomicnumber greater than 30, but less than 72 are optionally included in thematerials of the invention. Particularly preferred radiopacifying metaloxides include lanthanum oxide, zirconium oxide, yttrium oxide,ytterbium oxide, barium oxide, strontium oxide, cerium oxide, andcombinations thereof. Other suitable fillers to increase radiopacity aresalts of barium and strontium especially strontium sulphate and bariumsulphate.

The heavy metal oxide or metal fluoride particles may be surfacetreated. If present, x-ray visible particles are typically present in anamount of 0.1 to 20 or 1 to 15 or 2 to 10 wt. % with respect to theweight of the whole composition. The curable composition described inthe present text comprises an initiator system. The initiator system istypically present in an amount of 0.1 to 5 or 0.2 to 4 or 0.5 to 3 wt.%. The initiator system comprises a photoinitiator and an organic dye.

The initiator system contributes to an efficient cure of the curablecomposition, controls light penetration and light scattering and thusmay have an impact on mechanical and aesthetic properties.

In certain embodiments, the photoinitiator(s) can be characterized by atleast one or more, sometimes all of the following parameters:

-   -   showing a radiation absorption band within a range of 200 to 500        or 300 to 450 nm;    -   having a slightly yellowish colour.

The photoinitiator should be able to start or initiate the curing orhardening reaction of the radiation curable component(s) being presentin the curable composition.

The photoinitiator typically shows a light absorption band in a wavelength range of 300 to 450 nm, preferably in the range of 350 to 420 nm.Suitable examples of photoinitiators typically contain a phosphine oxidemoiety.

Examples of light curing initiator components include the class ofacylphosphine oxides, as described in e.g. in U.S. Pat. No. 4,737,593(Elrich et al.)

Such acylphosphine oxides are of the general formul:a

(R⁹)₂—P(═O)—C(═O)—R¹⁰

wherein each R⁹ individually can be a hydrocarbyl group such as alkyl,cycloalkyl, aryl, and aralkyl, any of which can be substituted with ahalo-, alkyl- or alkoxy-group, or the two R9 groups can be joined toform a ring along with the phosphorous atom, and wherein R¹⁰ is ahydrocarbyl group, an S—, O-, or N-containing five- or six-memberedheterocyclic group, or a —Z—C(═O)—P(═O)— (R⁹)₂ group, wherein Zrepresents a divalent hydrocarbyl group such as alkylene or phenylenehaving from 2 to 6 carbon atoms.

Preferred acylphosphine oxides are those in which the R⁹ and R¹⁰ groupsare phenyl or lower alkyl- or lower alkoxy-substituted phenyl. By “loweralkyl” and “lower alkoxy” is meant such groups having from 1 to 4 carbonatoms.

Tertiary amine reducing agents may be used in combination with anacylphosphine oxide. Illustrative tertiary amines include ethyl4-(N,N-dimethylamino)benzoate and N,N-dimethylaminoethyl methacrylate.

Commercially-available phosphine oxide photoinitiators capable offree-radical initiation when irradiated at wavelengths of greater than400 nm to 1,200 nm include a 25:75 mixture, by weight, ofbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide and2-hydroxy-2-methyl-1-phenylpropan-1-one (previously known as IRGACURE™1700, Ciba Specialty Chemicals),2-benzyl-2-(N,N-dimethylamino)-1-(4-morpholinophenyl)-1-butanone(previously known as IRGACURE™ 369, Ciba Specialty Chemicals),bis(η5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl)titanium (previously known as IRGACURE™ 784 DC, Ciba SpecialtyChemicals), a 1:1 mixture, by weight, ofbis(2,4,6-trimethylbenzoyl)phenyl phosphine oxide and2-hydroxy-2-methyl-1-phenylpropane-1-one (previously known as DAROCUR™4265, Ciba Specialty Chemicals), and ethyl-2,4,6-trimethylbenzylphenylphosphinate (LUCIRIN™ LR8893X, BASF Corp., Charlotte, NC),2,4,6-trimethylbenzoyldiphenyl-phospine oxide (LUCIRIN™ TPO).

Exemplary UV initiators include 1-hydroxycyclohexyl benzophenone(available, for example, under the trade designation “IRGACURE 184” fromCiba Specialty Chemicals Corp., Tarrytown, NY),4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl) ketone (available, forexample, under the previous trade designation “IRGACURE 2529” from CibaSpecialty Chemicals Corp.), 2-hydroxy-2-methylpropiophenone (available,for example, under the previous trade designation “DAROCURE D111” fromCiba Specialty Chemicals Corp. andbis(2,4,6-trimethylbenzoyl)-phenylposphineoxide (previously known as“IRGACURE 819” from Ciba Specialty Chemicals Corp.). The most preferredacylphosphine oxide is bis(2,4,6-trimethylbenzoyl)phenyl phosphine oxide(OMNIRAD™ 819, IGM Resin B.V., Waalwijk, NL).

The photoinitiator(s) is typically present in the following amounts:

-   -   Lower amount: at least 0.01 or at least 0.05 or at least 0.1 wt.        %;    -   Upper amount: at most 3 or at most 2 or at most 1.5 wt. %;    -   Range: 0.01 to 3 or 0.05 to 2 wt. % or 0.1 to 1.5 wt. %;    -   wt. % with respect to the weight of the whole composition.

The polymerizable composition described in the present text alsocomprises one or more organic dye(s).

The nature and structure of the organic dye(s) is not particularlylimited unless the desired result cannot be achieved.

It was found that by adding an organic dye, the ability of thepolymerizable composition described in the present text to absorbradiation can be enhanced.

In addition, it was found that adding an organic dye contributes tosupress or to lower the transmission of scattered light in thepolymerizable composition. This often helps to improve the accuracy ordetail resolution of the surface of the 3-dimensional article obtainedfrom the additive manufacturing process.

In certain embodiments, the organic dye(s) can be characterized by atleast one, more, of all of the following parameters:

-   -   a) having a light absorption band within a wave length range of        350 to 420 nm;    -   b) not having a light absorption band in the wave length range        of 400 to 800 nm;    -   c) comprising a terephthalate moiety.

The combination of parameters a) and b) is sometimes preferred.

Organic dyes which can be used include those containing a moietyselected form terephthalate groups and/or aromatic (hetero) cycles orother systems with delocalized pi-electrons. In particular dyes usefulfor colouring food were found to be useful.

Dye(s) which can be used include Lumilux™ Blue, Lumilux™ Yellow(Honeywell) and mixtures thereof.

If present, the organic dye(s) is present in the following amounts:

-   -   Lower amount: at least 0.001 or at least 0.002 or at least 0.005        wt. %;    -   Upper amount: at most 0.5 or at most 0.2 or at most 0.1 wt. %;    -   Range: 0.001 to 0.5 or 0.002 to 0.2 or 0.005 to 0.1 wt. %;        wt. % with respect to the weight of the whole composition.

The curable composition described in the present text may also compriseone or more additive(s).

Additive(s) include stabilizer(s), pigment(s), and mixtures thereof.

There is no need for additives to be present, so no additive(s) might bepresent at all. However, if present, they are typically present in anamount which is not detrimental to the intended purpose.

According to one embodiment, the curable composition comprises one ormore stabilizer(s).

The nature and structure of the stabilizer(s) is not particularlylimited, either, unless the desired result cannot be achieved.

A stabilizer may extend the shelf life of the polymerizable composition,help prevent undesired side reactions, and adjust the polymerizationprocess of the radiation curable component(s) present in the curablecomposition.

Adding one or more stabilizer(s) to the polymerizable composition mayfurther help to improving the accuracy or detail resolution of thesurface of the article to be produced.

In particular it was found that adding stabilizer(s) to thepolymerizable composition described in the present text may help toenhance the resolution and accuracy of the SLA process by attenuating oravoiding unwanted scattering effects.

Stabilizer(s) which can be added, include free radical scavengers suchas substituted and/or unsubstituted hydroxyaromatics (e.g. butylatedhydroxytoluene (Ionol), hydroquinone, hydroquinone monomethyl ether(MEHQ), 3,5-di-tert-butyl-4-hydroxyanisole(2,6-di-tert-butyl-4-ethoxyphenol), 2,6-di-tert-butyl-4-(dimethylamino)-methylphenol or 2,5-di-tert-butyl hydroquinone,2-hydroxy-4-methoxybenzophenone (UV-9),2-hydroxy-4-n-octoxybenzophenone, phenothiazine, p-methoxyphenole (MOP),2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO) and HALS (hinderedamine light stabilizers) and mixtures thereof.

If present, the stabilizer(s) is present in the following amounts:

-   -   Lower amount: at least 0.001 or at least 0.002 or at least 0.005        wt. %;    -   Upper amount: at most 0.5 or at most 0.2 or at most 0.1 wt. %;    -   Range: 0.001 to 0.5 or 0.002 to 0.2 or 0.005 to 0.1 wt. %;        wt. % with respect to the weight of the curable composition.

The curable composition described in the present text may comprisepigment(s).

If the curable composition is to be used in the dental or orthodonticfield, the pigment(s) should not negatively affect the aesthetics of thecured material.

Ideally, the cured composition should be tooth coloured, e.g. match witha colour of the Vita™ tooth colour guide.

Pigment(s) which might be present include titania, iron oxides andmixtures thereof,

If present, the pigment(s) is present in the following amounts:

-   -   Lower amount: at least 0.001 or at least 0.005 or at least 0.01        wt. %;    -   Upper amount: at most 0.02 or at most 0.05 or at most 0.5 wt. %;    -   Range: 0.001 to 0.5 or 0.005 to 0.05 wt. %;        wt. % with respect to the weight of the curable composition.

The curable composition described in the present text does not containsoftener(s) in an amount above 5 wt. % or above 3 wt. % with respect tothe weight of the whole composition.

During an autoclave process softeners might migrate from the curedcomposition. This may have a negative impact on the desired propertiesof the cured composition. Further, the curable composition described inthe present text does typically also not contain fumed silica, thesurface of which has been treated with hydrophilic and/or polymerizablesilanes in an amount which may negatively affect the viscosity and/orprintability. The amount of fumed silica the surface of which has beentreated with polymerziable silanes is typically not more than 2 wt. % or1.5 wt. % or 1 wt. % with respect to the weight of the wholecomposition.

The curable composition described in the present text can be produced bycombining and mixing the components of the composition. If desired, aspeed mixer can be used.

Typically, the radiation curable components are provided first and theother components are added thereto.

Due to the presence of a photoinitiator, the production process istypically carried out under save-light conditions.

During storage, the composition described in the present text istypically packaged in a suitable packaging device.

The curable composition described in the present text is typicallystored in container. Suitable containers include vessels, foil bags,cartridges, etc.

The volume of the respective containers is not particularly limited, butis typically in a range of 10 to 200,000 ml or from 500 to 10,000 ml.

The curable composition described in the present text can also beprovided as a kit of parts comprising the curable composition and aninstruction of use.

The instruction of use typically describes under what conditions thecurable composition should be used.

The invention also relates to a cured article obtainable or obtainedwhen curing the curable composition described in the present text.

As the cured article is based on the curing of the curable compositiondescribed in the present text, the cured article has basically oressentially the same chemical composition.

The cured article has typically the following properties alone or incombination:

-   -   flexural strength: 50 to 200 MPa or 80 to 150 MPa determined        according to ISO 4049:2009 using a test bar having the        dimensions 6*4*25 mm, while 6 mm is the width of the test bar;    -   E-modulus 1,000 to 4,000 MPa determined according DIN EN        843-2:2007 using the flexural strength method, while calculation        of the modulus is done in the range of 20% and 50% of maximum        force of the samples;    -   impact strength: 5 to 15 kJ/m2 determined according to DIN        53453:175-05;    -   abrasion: less than 20 or less than 15 or less than 10 mm³        (determined as described in the example section);    -   being autoclaveable without a significant change (e.g. +/−10%)        of mechanical properties;    -   being tooth coloured.

The combination of the following features is often preferred: highimpact strength combined with low abrasion.

A sufficient flexural strength is beneficial because the material of thecrown will not break easily.

A sufficient low E-modulus is beneficial because the material of thecrown has a sufficient flexibility.

A sufficient impact strength is beneficial because the material of thecrown has a high toughness and can resist to fracture

A sufficient abrasion is beneficial because the crown will not abradeand maintain its anatomical shape during chewing. According to oneembodiment, the cured article is a dental article.

Dental article can have different shapes, like the shape of a dentalcrown, bridge, inlay, onlay, veneer, table-top or parts thereof. Atable-top refers to a crown preparation where only the upper part (i.e.the chewing surface) of the crown is prepared. A table-top is sometimesalso referred to as “occlusion cap”.

According to a preferred embodiment, the dental article has the shape ofa preformed dental composite crown. The shape of a dental compositecrown is typically characterized as follows:

The preformed crown has a top surface and depending buccal, respectivelylabial, mesial, distal, lingual, respectively palatinal side surfaces.

The side surfaces are connected to each other and form a crown cervix.The lower region of the crown cervix forms the crown margin or crownrim.

The preformed dental composite crown has an outer and an inner surface.The inner surface is the surface to be attached to a prepared dentaltooth.

The wall thickness of the preformed crown at the crown cervix (in adistance of 1 mm from the crown margin) is equal to or lower than 0.8 orequal to or lower than 0.7 or equal to or lower than 0.6 mm or in arange of 0.1 to 0.8 mm or 0.1 to 0.7 mm or 0.1 to 0.6 mm or 0.1 to 0.5mm.

The wall thickness of the top surface (occlusal and/or distal) of thepreformed crown is typically in the range of 0.15 mm to 1.5 mm or in therange of 0.4 mm to 1.0 mm.

At least two of the opposing and depending side surfaces of thepreformed dental composite crown have a concave shape, preferably thebuccal and lingual side surfaces. That is, the side walls of thepreformed crown have a curved shape and thus provide an undercut in theregion of the crown cervix.

If desired, the dimension of the undercut U in mm can be calculated bythe formula U=D2−D1, wherein D1 is the distance of the opposing innerside walls having a concave shape of the preformed crown measured 1 mmabove the crown cervix, if the preformed crown is cut into halves andwherein D2 is the maximum distance of said opposing inner side walls ofthe crown measured parallel to D1.

In one embodiment, the shape of the preformed dental composite crown isfurther characterized by either of the following features alone or incombination:

The wall thickness of the side surfaces of the crown is typically notlarger than 0.7 mm or 0.6 mm or 0.5 mm or 0.4 mm.

According to one embodiment, the wall thickness of the side surfaces ofthe preformed crown is in a range of 0.1 mm to 0.7 mm. According toanother embodiment, the wall thickness of the side surfaces of thepreformed crown is in a range of 0.1 mm to 0.6 mm. According to anotherembodiment, the wall thickness of the side surfaces of the preformedcrown is in a range of 0.1 mm to 0.5 mm. According to a furtherembodiment, the wall thickness of the side surfaces of the preformedcrown is in a range of 0.1 mm to 0.4 mm.

Such a shape is described e.g. in EP application number EP 16158959.3(filed Mar. 7, 2016). The content of this document is herewithincorporated by reference.

The preformed dental composite crown has typically two kinds ofsurfaces: an outer surface being visible after fixation of the dentalarticle in the mouth of a patient and an inner surface becominginvisible after fixation of the dental article in the mouth of apatient.

According to one embodiment, the inner surface of the preformed dentalcomposite crown is roughened (e.g. by sandblasting) and/or has retentionelements. This feature may help to enhance adhesion of the crown to thesurface of a prepared dental tooth.

According to one embodiment, the outer surface of the preformed dentalcomposite crown is polished.

During storage the cured article is typically provided in a suitablepackaging device.

Suitable packaging devices include sealed blisters, plastic cases, traysor refill boxes.

The cured article described in the present text can be obtained bypolymerizing the curable composition described in the present text.

The polymerizing can be done by applying an additive manufacturingtechnique.

Such a technique is in particular useful, if the production ofindividualized articles is desired.

The polymerizable composition described in the present text can beprocessed as construction material in an additive manufacturing process,in particular in a stereolithography process (SLA).

According to one embodiment, the additive manufacturing processcomprises the steps of

-   -   providing a layer of the construction material on a surface,    -   radiation curing those parts of the layer of construction        material which will belong to the 3-dim article to be produced,    -   providing an additional layer of the construction material in        contact with the radiation cured surface of the previous layer,    -   repeating the previous steps until a 3-dim article is obtained.

Such a process comprises the step of applying radiation to the surfaceof a radiation curable material, wherein the radiation is applied onlyto those parts of the surface which will later form a part of thearticle to be produced.

Radiation can be applied by using e.g. a laser beam or by mask-imageprojection. Using a mask-image projection based stereolithographieprocess (MIP-SL) is sometimes preferred, as it allows a more rapidmanufacturing of the article.

A MIP-SL process can be described as follows:

-   -   i. A three-dimensional digital model of the article to be        produced is provided.    -   ii. The three-dimensional digital model is sliced by a set of        horizontal planes.    -   iii. Each thin slice is converted into a two-dimensional mask        image.    -   iv. The mask image is then projected with the aid of a radiation        source onto the surface of the radiation curable material being        located in a building platform (e.g. having the shape of a vat).    -   v. The radiation curable material is only cured in those regions        which are exposed.    -   vi. The building platform containing the radiation curable        material or the layer of cured material is moved relative to the        radiation source, wherein a new layer of radiation curable        material is provided being in contact with the layer of the        cured material produced in the previous step.    -   vii. Steps (iv) to (vi) are repeated until the desired article        is formed.

Projecting the mask image on the radiation curable material can be doneeither top-down or bottom-up with respect to the orientation of the vat.Using the bottom-up technique can be beneficial as less radiationcurable material is needed.

In this process, the radiation cured layer is formed on the bottom ofthe vat, which is transparent.

It was found that the polymerizable composition described in the presenttext is in particular useful for processing it in a mask-imageprojection stereolithography process using the bottom-up projectiontechnique.

Suitable process parameters for an SLA process include:

-   -   Wavelength of radiation: 350 to 420 nm;    -   Curing time: 0.5 to 20 sec.;    -   Layer thickness: 1 to 100 μm.

After processing the curable composition to form a 3-dim article, the3-dim article is typically removed from the device used for conductingthe additive manufacturing process.

If desired, the surface of the 3-dim article is cleaned, e.g. by rinsingthe 3-dim article with a solvent.

Suitable solvents include either low boiling alcohols as described inthe present text (e.g. an alcohol having a boiling point below 100° C.;like methanol, ethanol, n- or iso-propanol) and mixtures thereof or highboiling solvents as described in the present text, preferably the samesolvent(s) being present in the sol, e.g. diethylene glycol ethyl ether.

If desired, the 3-dim article can be post-cured by applying radiation orheat. Such a step may help to improve the stability of the 3-dimarticle.

If applied, the post-curing step can be characterized by at least one,or all of the following features:

-   -   applying radiation with wavelength of 350 to 450 nm;    -   applying a heating step of 30 to 120 or from 40 to 80° C.

Thus, a suitable process of processing the curable composition describedin the present text may comprise the following steps:

-   -   a) providing the curable composition as described in the present        text,    -   b) producing a cured article precursor with a 3d-printer having        a vat by radiation curing the curable composition, the cured        article having an outer and an inner surface,    -   c) removing the cured article precursor from the vat of the        3d-printer,    -   d) cleaning the cured article precursor,    -   e) post-curing the cured article precursor to obtain a cured        article,    -   f) optionally removing any support structures, if present    -   g) optionally polishing at least parts of the outer surface of        the cured article,    -   h) optionally sandblasting at least parts of the inner surface        of the cured article.

If desired, step c) can also be accomplished after step d) or after stepe).

According to one embodiment, the cured article has the shape of apreformed dental composite crown.

The cured article precursor may comprise supporting elements, whichsupport and carry the cured article precursor during the 3d-printingprocess. If present, the supporting elements are removed later in theprocess, e.g. after step c), d) or e).

The invention also relates to a kit of parts comprising at least twocured articles described in the present text, wherein the cured articleshave the shape of a dental crown.

A kit can comprise up to 200 or up to 150 differently shaped preformeddental composite crowns.

A kit can comprise each of the differently shaped preformed dentalcomposite crowns in an amount up to 10 or up to 8 different sizes.

The preformed dental composite crowns can have the shape of an anterioror posterior tooth.

Typically, the preformed dental composite crowns are provided indifferent tooth colors. Tooth colors are typically classified accordingto the Vita′ color code.

The invention also relates to a kit of parts comprising at least onecured article having the shape of a dental article obtained by radiationcuring the curable composition described in the present text and one ormore of the following items:

-   -   dental cement,    -   instruction for use.

Thus, the kit of parts can also comprise a dental cement suitable forsecurely fixing the preformed dental composite crown to a prepared toothsurface.

Suitable dental cements are glass ionomer cements and in particularresin modified glass ionomer cements. Glass ionomer cements typicallycontain the following components: acid-reactive filler, polyacid, water,and complexing agent, but no radiation curable components.

Glass ionomer cements are typically provided as a kit of part comprisinga liquid part and a powder part. The two parts have to be mixed beforeuse.

The powder part typically comprises an acid-reactive inorganic filler(e.g. a fluoro alumosilicate glass, FAS glass).

The liquid part typically comprises a polyacid, water and a complexingagent (e.g. tartaric acid).

Glass ionomer cements are commercially available (e.g. Ketac™ Cem; 3MOral Care).

The glass ionomer cement can also be provided as a kit of partscomprising two pastes A and B to be mixed before use.

According to a preferred embodiment, the kit of parts containing thepreformed dental composite crowns comprises a resin modified glassionomer cement (RM-GIZ).

Resin modified glass ionomer cements typically contain the followingcomponents: acid-reactive filler, polyacid, water, complexing agent,radiation curable components, initiator.

Suitable radiation curable components typically contain (meth)acrylatemoieties.

Resin modified glass ionomer cements are provided as kit of parts aswell, either as powder/liquid system or paste/paste system.

The powder part typically comprises acid-reactive inorganic filler(s)(e.g. a fluoro alumosilicate glass, FAS glass) and initiator components.

The liquid part typically comprises polyacid, water, (meth)acrylates andinitiator components.

Resin modified Glass ionomer cements are commercially available (e.g.Ketac™ Cem Plus; 3M Oral Care).

It was surprisingly found that in particular a RM-GIZ is suitable forsecurely fixing the preformed dental composite crown to the surface of aprepared tooth.

Even, if a little bit more expensive than classical glass ionomercements, RM-GIZ can be obtained at a lower expense compared toself-adhesive resin cements (e.g. RelyX™ Unicem or adhesive resincements (e.g. RelyX™ Ultimate; 3M Oral Care).

According to one embodiment, the cured article described in the presenthas the shape of a dental article, in particular the shape of apreformed dental composite crown.

The preformed dental composite crown described in the present text isused for treating a dental tooth, in particular in the pediatric field.

Such a method comprises the step of fixing the preformed dentalcomposite crown as described in the present text to the surface of aprepared tooth in the mouth of a patient by using a dental cement asdescribed in the present text, in particular a resin modified glassionomer cement.

If desired, the surface of the preformed dental composite crown which isto be attached to the surface of the prepared tooth (“inner surface”)can be roughened before the dental cement is applied. Roughening can bedone e.g. sandblasting.

Roughening the surface can be useful to even further improve thefixation of the preformed crown to the prepared surface of the tooth tobe treated.

If desired, the shape of the preformed dental composite crown can befurther adapted by a cutting or grinding.

According to one embodiment the curable composition as described in thepresent text comprises:

-   -   polymerizable (meth)acrylate(s) not comprising a urethane moiety        -   comprising at least two (meth)acrylate moieties,        -   having a molecular weight of 170 to 3,000 g/mol, and        -   being present in an amount of 40 to 85 wt. %,    -   the polymerizable urethane(meth)acrylate(s)        -   comprising at least three urethane moieties,        -   having a molecular weight of 450 to 3,000 g/mol, and        -   being present in an amount of 1 to 35 wt. %,    -   nanocluster        -   comprising Si or Zr based particles, and        -   being present in an amount of 5 to 40 wt. %,    -   optionally fumed silica        -   being present in an amount of 0.5 to 7 wt. %,    -   X-ray visible particles        -   being present in an amount of 0 to 15 wt. %    -   photoinitiator        -   comprising a phosphine oxide moiety, and        -   being present in an amount of 0.01 to 3 wt. %,    -   stabilizer being present in an amount of 0.001 to 0.5 wt. %,    -   organic dye in an amount of 0.001 to 5 wt. %,    -   the curable composition not comprising        -   softener in an amount of more than 5 wt. %,    -   wt. % with respect to the weight of the whole composition.

The components used for producing the curable composition and inparticular the dental articles described in the present text should besufficiently biocompatible, that is, the composition should not producea toxic, injurious, or immunological response in living tissue.

The complete disclosures of the patents, patent documents, andpublications cited herein are incorporated by reference in theirentirety as if each were individually incorporated. Variousmodifications and alterations to this invention will become apparent tothose skilled in the art without departing from the scope and spirit ofthis invention. The above specification, examples and data provide adescription of the manufacture and use of the compositions and methodsof the invention. The invention is not limited to the embodimentsdisclosed herein. One skilled in the art will appreciate that manyalternative embodiments of the invention can be made without departingfrom the spirit and scope of thereof.

The following examples are given to illustrate the invention.

Examples

Unless otherwise indicated, all parts and percentages are on a weightbasis, all water is de-ionized water, and all molecular weights areweight average molecular weight. Moreover, unless otherwise indicatedall experiments were conducted at ambient conditions (23° C.; 1013mbar).

Methods Viscosity

The viscosity has been measured using a Physica Rheometer MCR 301 devicewith a plate-plate system (diameter 15 mm) and a slit of 0.2 mm. Theviscosity values (Pas) were recorded at 23° C. for a shear rate of 1s⁻¹.

Flexural Strength

If desired, flexural strength can be determined by conducting a threepoint flexural strength test according to ISO 4049:2009 using testspecimen having the size 4*6*25 mm. Flexural strength is given in [MPa].

E-Modulus

The E-Modulus was determined according to DIN EN 843-2:2007 using a testbar having the dimensions 6*4*25 mm, with 6 mm being the width of thesample. The E-Modulus was determined between the range of 20% and 50% ofthe maximum force of the test specimen. E-Modulus is given in [GPa].

Impact Strength

If desired, the impact strength can be determined according to DIN53453:1975-05 (Charpy) using test samples having the dimensions 4*6*50mm, using a Zwick 5102 pendulum set up with a 0.5 J pendulum and using aspan of 42 mm. Impact strength is given in [kJ/m2].

Abrasion

If desired, abrasion [mm³] can be determined as follows: Abrasion testswere performed at specific specimens with a slope of 30°. For thatpurpose the materials were filled into the depression of M12Inbus-screws and cured according to the manufacturers' instructions.

The specimens were flat grinded using a 75 μm diamond saw and stored indistilled water for 4 days at 36° C. Then chewing simulation was startedapplying the following conditions:

Chewing force: 80 N; Lateral movement: 4 mm; Sliding movement: 10 mm;Antagonist: steatite ball; Number of chewing cycles: 1,200,000;Thermocycles (5/55° C.): 5,000.

After conducting the chewing simulation abrasion was determined bymeasuring the loss of volume using a laser scanning microscope VK-X200(Keyence Company).

Further information about the abrasion test can be found in M.Rosentritt et al., Materialprüfung 39 (1997), p. 77-80.

Evaluation of Printability

Two different printing setups were used, each containing at least 2article precursors on the printing platform:

-   -   Printing Setting 1: distance of the article precursors was 2 mm        or less.    -   Printing Setting 2: distance of the article precursors was at        least 3 mm.

A lower distance between the article precursors allows the simultaneousprinting of a higher number/volume of articles on a given printingplatform and thus a more efficient manufacturing process.

After the printing process, the printed article precursors were visuallyevaluated and classified as “YES”, if the geometrical dimensions of theprinted article precursor matched with the geometrical dimensions of therespective STL file and “NO”, if the geometrical dimensions of theprinted article precursor did no match with the geometrical dimensionsof the respective STL file.

Materials

TABLE 1 Zr/Si Nanocluster aggregated nanoparticles; nanoclusters; U.S.Pat. No. 6,730,156 B1, filler column 25, Preparatory Example A; theobtained filler particles were surface treated according to PreparatoryExample B of U.S. Pat. No. 6,730,156 B1. HDK ™ H-2000 fumed silicafiller; surface modification: —OSi(CH₃)₃; agglomerated nanoparticlesAerosil ™ R711 fumed silica filler; surface modification: methacrylsilane SG-YBF100 ytterbium fluoride powder; filler D-Zethacrylateethoxylated Bisphenol A dimethacrylate; polymerizable methacrylate DESMAurethane(meth)acrylate; polymerizable Example 1 of EP 2 167 013methacrylate B1 (page 20) GDMA Glycerol dimethacrylate Ionol ™2,6-ditert.butyl-4-methylphenol; stabilizer Z-Acetate ethoxylatedBisphenol A diacetate; softener TEGDMA Triethylenglycole dimethacrylateLucirin ™ TPO Photoinitiator Ircacure ™ 819 Photoinitiator Lumilux ™Blau Organic dye LZThe compositions outlined in Table 2 were prepared. The amount of thecomponents is given in parts by weight (pbw):

TABLE 2 Ex1 (CE) Ex2 (CE) Ex3 (CE) Lucirin TPO — — — Ircacure 819 —0.54000 0.54000 Lumilux Blau LZ 0.00136 0.00136 0.00136 Cu-Procetonat0.00273 — — Amine-HCl 0.17273 — — Ionol 0.02727 0.02727 0.02727D-Zethacrylat 39.96591 39.96727 39.96727 DESMA 4.37545 4.37545 4.37545HDK H-2000 4.54545 4.54545 4.54545 SG-YBF 100 2.27273 2.27273 2.27273Aerosil R711 1.81818 1.81818 1.81818 Zr/Si-Nanocluster 38.63636 38.6363638.63636 TBPIN 0.02727 — — BZPBS 0.90909 — — Z-Acetate 7.24545 7.24545 —Ex4 (CE) Ex5 (CE) Ex6 (IE) Ex7 (CE) Lucirin TPO — 0.54000 0.540000.54000 Ircacure 819 0.54000 — — — Lumilux Blau LZ 0.00136 0.001360.00136 0.001492 Cu-Procetonat — — — — Amine-HCl — — — — Ionol 0.027270.02727 0.02727 0.0497 D-Zethacrylat 39.96727 39.96727 39.96727 0.1477GDMA — — — 29.7186 DESMA 4.37545 4.37545 4.37545 19.8124 HDK H-20004.54545 4.54545 1.70000 3.9794 SG-YBF 100 2.27273 2.27273 2.27273 2.4865Aerosil R711 1.81818 1.81818 — 0.9946 Zr/Si-Nanocluster 18.2000018.20000 18.20000 42.2705 TBPIN — — — — BZPBS — — — — Z-Acetate — — — —CE: Comparative Example; IE: Inventive Example

The composition of Ex1 essentially corresponds to the compositiondescribed in example 1 of WO 2015/006087 A1 and functions as acomparative example (CE).

In the composition of Ex2 the redox initiator system of Ex1 has beenreplaced by a photoinitiator system.

In the composition of Ex3 the softening component (Z-Acetate) has beenomitted.

In the composition of Ex4 the amount of filler has been reduced.

In the composition of Ex5 the photoinitiator system has been switched.

In the composition of Ex6 the amount of methaycrylsilane treated silicahas been reduced.

The composition of Ex7 essentially corresponds to the compositiondescribed in example 3 of US 2016/0136059 A1 (Hecht et al.) except thata photo initiator system was used instead of a redox curing initiatorsystem.

General Processing of Curable Compositions

For the curable compositions Ex1 given in Table 2, the components wereprovided and mixed using a kneader to obtain homogenous paste A andpaste B. The pastes were filled in a dual chamber cartridge with avolume of Paste A to Paste B of 10:1 (SulzerMixpac). The composition wasdispensed through a static mixing tip (SulzerMixpac) by using a manuallydriven gear into moulds having the geometry of 4 mm×6 mm×25 mm, removedafter 1 h and stored into de-ionized water for 24 h.

For the curable compositions of Ex2, Ex3, Ex4, Ex5 and Ex6, thecomponents were provided and mixed using a kneader to obtain ahomogenous paste. The paste was poured into the working tray of acommercially available DLP printer (Rapidshape HA30; Heimsheim,Germany).

The pre-processing data (STL-file; shape of 3-dim. Object; 4 mm×6 mm×25mm) was loaded into the printer.

The following printing conditions were applied:

-   -   curing light wavelength: 383 nm light;    -   exposure time: 11 sec;    -   layer thickness: 50 μm;    -   printing protocol: using the standard parameter set for material        GP101 (Software: Netfabb Professional for Rapidshape 5.2 64        bit).

The article precursors were finished by applying the following steps:

-   -   removing the article precursor from the building platform    -   cleaning the article precursor for 5 min in isopropanol using        ultrasonic device (Fa. BANDELIN electronic GmbH & Co. KG, DT 100        H)    -   light curing the article precursor for 900 sec under argon        conditions.

The obtained cured articles were tested with respect to viscosity andprinting properties as outlined in Table 3. A viscosity above 150 Pa*swas considered as too high for a proper processing.

TABLE 3 Ex1 Ex2 Ex3 Ex4 Ex5 Ex6 Ex7 Radiation curability No Yes Yes YesYes Yes Yes Printing Setting 1 n.d. No No No No Yes n.d. PrintingSetting 2 n.d. n.d. n.d. Yes n.d. Yes n.d. Viscosity in [Pas] n.a. 217357 195 202 18.2 268 at shear rate 1 [s⁻¹] n.d. = not determined

Further, the obtained cured articles were tested with respect to themechanical properties as outlined in Table 4 before and after havingconducted an autoclave process simulated by applying the followingconditions: placing the cured article in an oven for 24 h at 120° C.

Ex1 Ex4 Ex6 inital values Elastic Modulus 3.1 3.8 2.7 in Gpa 24 h-120°C. Elastic Modulus 3.9 3.6 2.8 in Gpa Delta in % Elastic modulus 20 −6 4

Due to the absence of a photoinitiator, the composition of Ex1 is notradiation curable and cannot be processed in an SLA process.

The composition of Ex2 contained a photoinitiator and can thus beprocesses in an SLA process.

However, the printability of the precursor articles using the PrintingSetup 1 was classified as not satisfying. The viscosity of the curablecomposition was too high.

The composition of Ex3 contained a photoinitiator but not a softener.Like the composition of Ex2, the printability of the precursor articlewas classified as not satisfying. The viscosity of the curablecomposition was too high.

The composition of Ex4 contained a lower amount of cluster filler wasused. The composition was classified as being autoclaveable. Further,the viscosity of the curable composition could be lowered, but theprintability of the precursor article using the Printing Setup 1 wasstill not satisfying.

The composition of Ex5 contained a different photoinitiator. Theprintability and viscosity of the composition did not significantlychange.

In the composition of Ex6 the amount of silica filler has been reduced.The viscosity of the curable composition dropped significantly. Theprintability using the Printing Setup 1 was classified as satisfying.Further the cured composition was also cured as being autoclaveable.

What is claimed is:
 1. A curable composition for producing dentalcomposite crowns, the composition comprising a resin matrix comprising:polymerizable (meth)acrylate(s) not comprising a urethane moiety,polymerizable urethane(meth)acrylate(s), wherein the polymerizable(meth)acrylate(s) not comprising an urethane moiety are used in excessover the polymerizable urethane(meth)acrylate(s), a filler matrixcomprising: nanocluster(s), optionally fumed silica in an amount below 8wt. %, the filler matrix being present in an amount of 5 to 45 wt. %, aninitiator system comprising: photoinitiator(s), organic dye(s), thecurable composition not comprising softener in an amount of more than 5wt. %, wt. % with respect to the weight of the whole composition, thecurable composition having a viscosity below 150 Pa*s at 23° C. and ashear rate of 1 s⁻¹.
 2. The curable composition as described in claim 1,the resin matrix being present in an amount of 40 to 85 wt. %, theinitiator system being present in an amount of 0.01 to 5 wt. %, wt. %with respect to the weight of the whole composition
 3. The curablecomposition as described in claim 1, being characterized by thefollowing features alone or in combination: curable by radiation havinga wavelength of in the range of 350 to 600 nm; viscosity: 1 to 100 Pa*sat 23° C. at a shear rate of 1 s⁻¹; pH value: 6 to
 8. 4. The curablecomposition as described in claim 1, the fumed silica, if present, beingcharacterized by the following features alone or in combination: BETsurface: 100 to 300 m2/g; being surface treated with an alkyl silane. 5.The curable composition as described in claim 1, the compositioncomprising: polymerizable (meth)acrylate(s) not comprising a urethanemoiety in an amount of 40 to 85 wt. %, polymerizableurethane(meth)acrylate(s) in an amount from 1 to 35 wt. %, nanoclusterin an amount of 5 to 40 wt. %, fumed silica in an amount of 0.5 to 5 wt.%, photoinitiator in an amount of 0.01 to 3 wt. %, organic dye in anamount of 0.001 to 0.5 wt. %.
 6. The curable composition as described inclaim 1, comprising in addition the following components alone or incombination: X-ray visible particles: preferably in an amount of 0.1 to5 wt. % stabilizer, preferably in an amount of 0.001 to 0.5 wt. %;pigments, preferably in an amount of 0.001 to 0.5 wt. %; wt. % withrespect to the weight of the whole composition.
 7. The curablecomposition as described in claim 1, the composition not comprising thefollowing components alone or in combination: redox initiator system;fumed silica surface treated with (meth)acrylate silanes in an amount ofmore than 2 wt. %; glass or glass ceramic particles in an amount of morethan 5 wt. % wt. % with respect to the weight of the whole composition.8. The curable composition as described in claim 1, being characterizedas follows: polymerizable (meth)acrylate(s) not comprising a urethanemoiety comprising at least two (meth)acrylate moieties, having amolecular weight of 170 to 3,000 g/mol, and being present in an amountof 40 to 85 wt. %, the polymerizable urethane(meth)acrylate(s)comprising at least three urethane moieties, having a molecular weightof 450 to 3,000 g/mol, and being present in an amount of 1 to 35 wt. %,nanocluster comprising Si or Zr based particles, and being present in anamount of 5 to 40 wt. %, optionally fumed silica being present in anamount of 0.5 to 7 wt. %, X-ray visible particles being present in anamount of 0 to 15 wt. % photoinitiator comprising a phosphine oxidemoiety, and being present in an amount of 0.01 to 3 wt. %, stabilizerbeing present in an amount of 0.001 to 0.5 wt. %, organic dye in anamount of 0.001 to 5 wt. %, the curable composition not comprisingsoftener in an amount of more than 5 wt. %, wt. % with respect to theweight of the whole composition.
 9. A cured article obtained byradiation curing the curable composition as described in claim 1 in itscured state.
 10. The cured article as described in claim 9, beingcharacterized by the following features alone or in combination:flexural strength: 50 to 200 MPa determined according to ISO 4049:2009;E-modulus: 1,000 to 4,000 MPa determined according to DIN EN 843-2:2007;impact strength: 5 to 15 kJ/m2 determined according to DIN 53453:175-05;abrasion: less than 20 mm³; being autoclaveable; being tooth coloured.11. The cured article as described in claim 9, having the shape of adental crown, bridge, inlay, onlay, veneer, table-top or part thereof.12. A kit of parts comprising at least two cured articles as describedin claim 9, the cured articles having the shape of a dental crown anddiffering from each other in the following features alone or incombination: size; colour; shape.
 13. A kit of parts comprising: atleast one cured articles as described in claim 9, the cured articleshaving the shape of a dental crown, and the following items alone or incombination: self-adhesive dental cement; instruction for use.
 14. Aprocess for producing a cured article, the process comprising the stepof processing the curable composition as described in claim 1 byapplying an additive-manufacturing technique.
 15. The process of claim14, the processing of the curable composition comprising the followingsteps: providing the curable composition, producing a cured articleprecursor with a 3d-printer having a vat, removing the cured articleprecursor from the vat of the 3d-printer, cleaning the cured articleprecursor, post-curing the cured article precursor to obtain a curedarticle, optionally polishing at least parts of the surface of the curedarticle.